The accumulation of ice on various surfaces of an aircraft can produce disastrous results. Accordingly, it is important for a pilot to know when ice starts to appear and to take measures to avoid serious consequences. For example, a pilot may turn on surface heaters, make changes in speed or elevation, changes in the angle of attack and/or seek the nearest airport before the problem becomes critical.
A U.S. Pat. No. 4,851,817 of Brossia et al. discloses a system for automatic and real time detection of water and icing on surfaces by monitoring variations in light energy transmitted through an optical fiber having a specially processed sensitive area probe. The sensitive area probe is positioned on, about or within the surface on which icing is to be detected. Because of differences in optical indices of refraction and energy absorption characteristic of air, water and ice, the presence of each of these at the process sensitive area will cause a proportional and characteristic attenuation of the light energy passing through the optical fiber. Changes in light energy transmission can be interpreted automatically to provide an indication of icing. A referenced optical circuit may be used to provide compensation for variations in input energy levels. Light energy of different wavelengths and energy levels may be used to compensate for or avoid interference with measurement by ambient lighting conditions or for the detection of other conditions and materials using the principle of characteristic absorption and resonance.
A more recent approach to an Ice Detector, Especially for Aircraft is disclosed by Michaoud et al. in U.S. Pat. No. 5,014,042. As disclosed therein, a source sends light towards a receiver through an optical channel, a part of which has an interface with the external environment. The light that reaches this interface is reflected toward the receiver when the external environment is in contact with air and is refracted toward the external environment in the presence of water or ice. In streaming down the interface, the rain creates a modulation, not created by ice in the signal of the receiver. The circuits down line of the receiver search for this modulation to determine whether the modifications of the signal of the receiver are due to rain or ice.
Notwithstanding the above, it is presently believed that there may be a large commercial market for an improved optical system and element for detecting ice and water on the surface of an aircraft in accordance with the present invention. A commercial market should develop because the improved systems and elements in accordance with the present invention warn a pilot of a hazardous icing condition, enables a pilot to distinguish between ice and water, as well as conditions when icing may be imminent. It is presently believed that such systems can be manufactured and sold at a competitive costs, can be readily installed on new aircraft as well as retrofitted on existing aircraft, are compact, durable, accurate and readily serviced. Further advantages will become evident from the following specification.